The present invention relates to a method for fabricating fine line electrical conductors.
Fine line electrical conductors are fabricated both on flexible cables and rigid backings, and are widely used in and with a wide variety of electronic equipment for land, sea, air and space applications.
Fabrication of such flexible and rigid devices is generally effected by preparing a fine line circuit on a substrate material, such as of Kapton, and then providing a laminate therefrom to seal the conductor lines. To provide electromagnetic protection, a shield layer is then deposited on the laminated structure. Both the layered constructions are also made for high density interconnections. Modern electro-optical devices demand high packaging density, which may be met by using multilayers and/or by reducing the line width of the conductors. A typical line width is 4 mil or more, with future needs being toward finer lines.
The major problems in producing such fine line cables are associated with the preparation of the conductor lines. Conventionally, interconnecting circuit patterns are formed by a photlithographic process and by a chemical etching process, which involve many steps. Since the handling in any one particular step can create defects, therefore, as the steps are increased, the overall yield correspondingly decreases.
Conductors and fine line applications must have fine geometry and minimum voltage drop, which requires a high aspect ratio, that is, a ratio of the conductor's thickness to its width. In chemical processes, which are subtractive processes in that material is removed from a conductive layer to produce the fine line conductors, undercutting becomes a common problem. As the aspect ratio descreases, undercutting becomes an even greater problem and eventually cannot be tolerated. As a result, undercutting is a prime reason for rejecting articles.
Additional sources of rejected cables result from openings in or short circuiting between fine line conductors and poor adhesion of photoresist and metallization on the base substrate to which the fine line conductors are secured. Openings in the circuit increase exponentially with a decrease in line width.
While both additive and subtractive processes require use of thin film stock, in the subtractive process special care must be used to prevent such defects as dents, scratches and creases which affect the production yield and product quality.
Access holes or vias are provided in the proporting dielectric film for access to solder, wire bonding and multilayer interconnects. When these holes are prefabricated in the film, the lack of mechanical support for the metal foil in these areas provides vulnerability to mechanical damage and to chemical attack during fabrication processing.
Such components as infrared detectors which are operated at cryogenic temperatures, require that one end of the interconnection cable for the detectors be at such a temperature, while the other end is at ambient temperature. This temperature differential or gradient creates a problem in heat loss by conduction. Thus, the thermal conductivity and electrical conductivity balance must be considered in the material selection.